The present invention is directed to a grinding aid composition and to an enhanced method of grinding slag by using said grinding aid in conjunction with a roll press. Specifically, the present invention is directed to the use of polyacrylic acid or its alkali metal salt as a means of enhancing the grinding efficiency of slag material by a roll press mill circuit.
Slag is a by-product commonly obtained from iron blast furnace operations. Conventionally, the blast furnace is charged with layers of iron ore, flux stone, fuel and other sources of iron oxide as part of a highly controlled metallurgical process. Heat and oxygen are forced into the furnace to achieve very high temperatures and molten iron is collected by tapping the bottom of the furnace. Molten slag which is formed just above the molten iron is also tapped out and withdrawn from the furnace where it is quenched with water to produce a wet granular slag material.
The granulated blast furnace slag is a non-metallic product consisting mainly of silicates and alumino silicates of calcium and other bases. ASTM C-989 provides specifications for granulated slag which can be used in concrete and mortar compositions and ASHTO-MR02 provides the specification for the ground product which can be formed from the granular slag and used as a component in blended cements (e.g. ASTM C-595 Standard Specifications for Blended Hydraulic Cements).
Blended cement compositions can be formed by replacing a portion (up to about 50 weight percent) of the hydraulic cement component of the composition with a ground powder slag product. The cement compositions of mortar (hydraulic cement, small aggregate such as sand, and water) and concrete (hydraulic cement, small aggregate, large aggregate such as stone, and water) generally exhibit enhanced later age strength when slag is present as part of the composition.
Granulated slag is normally treated by ball mill or roll press to provide the powdered product. In the ball mill operation, the granules are treated by continuous random striking with the ball elements of the mill to break up the granules into the desired powder. The ball mill operates at higher efficiency when there is an agent (commonly called "grinding aid") present in the mill which causes the formed particles to remain dispersed within the ball mill. Thus, compounds such as lignosulfonates, triethanolamine and the like have been used in ball mill operations.
The roll press performs in a distinctly different mechanism from that of the ball mill. The slag granules are fed to the nip of a pair of rollers. The granules are subjected to a single squeezing force which occurs while the granules pass between the rollers. The rollers squeeze the granules causing them to fracture into very small particles as well as to cause fracturing of the granules which will completely disassociate upon subsequent subjection to a deagglomerator.
One problem associated with the roll press is the rate at which the feed granules are capable of passing between the rollers. Small particles (from recycle stream) have little problem passing through but larger granules (from raw feed) tend to be retained above the nip of the rollers. It is generally known that the addition of small amounts of water, such as about 3 weight percent based on the total feed, provides a means to aid in causing the large granules to enter the nip area more readily. It is believed that the water causes the granules and particles to adhere and interact to draw them through the nip area.
The final product must be a dry powder and, therefore, the water must be removed at the finishing end of the process. The use of water has the defects of requiring large amounts of energy to drive off the water from the powder product; reduces the efficiency of the deagglomerator and causes clogging of dust collectors which are associated with the process. Thus, it is desired to provide a composition which provides a means of enhancing the efficiency of slag pulverization by a roll mill while reducing the water required.